Are you going to race these birds?
I personally don't start road training until about 7 or 8 weeks before the first race. 6 weeks is actually good, but I like to account for a week of bad weather which can put you way behind.
Also depends on how far your first race is. Mine was always 160 to 200 miles, but this year, we're flying from a different direction so my first race will be around 120, do I don't feel the need to train as hard or as far.
There IS such a thing as training too much and burning them out, IMO........as long as they're routing and flying good around the loft, I wouldn't push it until you get closer to racing. You'll find (at least I do) that the older the birds get, the less they fly and want to hang out and chase the hens. That's when you've got to get them on the road...........